Piezoelectric elements produce electrical potential, when a mechanical force is used to compress them. In comparison to conventional electromagnetic generators, piezoelectric elements have lower energy losses in heat. Materials possessing the piezoelectric effect capability include: quartz, tourmaline, Plumbum Zirconate Titanate (PZT) and Plumbum Magnesium Niobate (PMN). PZT and PMN are ceramic materials, which have shown high efficiency in generating electrical output, when mechanical stress is applied. Several configurations of piezoelectric ceramic materials are capable of producing electrical potential. One example is a stack of thin layers of rectangular or circular shape, separated by thin metallic electrodes, bonded together. The typical layer thickness is 1 mm or less per piezoelectric layer. These stacks can withstand the pressure of several tons. Piezoelectric stacks are mounted by gluing them on metal or ceramic surfaces by cold or hot curing epoxy, respectively. A guiding casing is essential when heavy loads move dynamically. This guiding casing usually includes a preload spring. The piezoelectric stacks in a guiding casing with a preload spring constitute a piezoelectric device. The stacks have their own characteristic stiffness. The preload spring must have 10% or less of the stack's stiffness for optimum performance. The maximum permissible forces depend on the operating frequency of the application in use and the maximum allowable displacement of the stack(s). A typical stack displacement is less than 1 mm. Each stack, when compressed, can generate more than 1000 Volts. The piezoelectric stacks can perform billions of cycles without any measurable wear. In practical terms their lifetime exceeds 30 years. Piezoelectric devices are commercially available from a few companies including PI Ceramic of Karlsruhe, Germany. The electrodes of these devices can be connected in series, which maximizes the combined voltage or in parallel, which, when connected to additional circuitry, the current is maximized. Combined schemes are possible. Advances in power electronics have allowed the electrical potential generated by piezoelectric stacks to deliver useful electrical power in AC or DC form, which can be stored in capacitors or be fed in the electricity grid.
An attempt for high power harvesting, utilizing piezoelectricity is described in U.S. Pat. No. 7,005,779. The generator described in this patent utilizes piezoelectric devices activated by the rotation of a cam with a special shape, which is rotated by an external source. Although the special shape of the cam compresses the piezoelectric stacks, the rotation of the cam applies also torque on the piezoelectric stacks, which may affect their stability.
The generator described herein, compresses piezoelectric devices hydraulically. The compressive force is only in one direction without involving rotation and torque, thus maintaining perfect stability of the piezoelectric stacks used.
An attempt for high power harvesting, utilizing the gravitational weight of traversing vehicles over piezoelectric devices, is described in US patent application No. 20090195226. This patent application utilizes boxes containing piezoelectric stacks, buried under the asphalt road. When a vehicle traverses over each box, activates the piezoelectric stacks inside the box. The box's width is similar to the width of a vehicle's tire. This way, all the piezoelectric stacks in the box are utilized, when the vehicle's tire passes over the box. If the box were wider than the tire, so as to fit more piezoelectric stacks, the top cover of the box would not efficiently distribute the same pressure to all piezoelectric stacks in the box. The generator described herein, has significant advantages over the “box method” of the patent application 20090195226. It allows for more piezoelectric stacks to be compressed in comparison with the “box method”. This is due to Pascal's principle of physics, which explains that the pressure generated by a crossing tire over the generator is transmitted undiminished to all the points of the hydraulic fluid included in the generator. This fluid is used to transfer the pressure to a compressing piezo-stacks surface. This surface can be substantially larger in comparison to the surface of the box.
Hydraulic methods have been employed to harness energy from renewable sources such as wind and ocean waves. The usual method involved is hydraulic motors. For example, U.S. Pat. No. 4,931,662 utilizes a hydraulic motor to convert the rise and fall of the ocean waves to rotation in order to rotate an electromagnetic generator. Hydraulic motors provide rotational kinetic energy. They use the non-compressible property of the liquids to multiply a torque instead of a linear force. Hydraulic press machines utilize the non-compressible property of liquids to multiply linear forces. Hydraulic press machines typically employ two hydraulic cylinders, one small and one large hydraulically coupled through tubing containing a liquid. A small force applied to the small cylinder can be multiplied many times by the large cylinder, due to Pascal's principle. However, there is a drawback involved: in order to multiply a force, the small hydraulic cylinder must be displaced a large distance in order to cause a considerable displacement of the large hydraulic cylinder. This is probably the main reason why the hydraulic press method is not used in electrical power generation. That is, the displacement of the large cylinder, as output, cannot be used to move considerably the rotor of a linear or rotational electromagnetic generator. However, the piezoelectric power generator, described herein, exploits fully the benefits provided by this hydraulic method turning the drawback to an advantage.